What Is a SQL-NS Application?

A SQL-NS application is a piece of software that gathers events and subscriptions and matches them to produce notifications. The kinds of events, subscriptions, and notifications, as well as the logic used to determine when a match occurs, are specific to each application. When you build an application, you specify these things for your application's domain. In this chapter, we'll do this for the music store application.

Whereas a SQL-NS instance is a unit of administration and deployment, a SQL-NS application is a unit of functionality. An application encapsulates event, subscription, and notification data, and the logic that relates them.

A complete SQL-NS application has the following functional parts:

• Event providers that submit events into the application

• A subscription management interface that adds, deletes, and modifies users' subscriptions

• A matching component that matches events with subscriptions and generates notifications

• Content formatters that transform raw notification data into a format presentable to the final recipient

• Delivery protocols that deliver formatted notifications

These functional parts (illustrated in Figure 5.1) are each responsible for handling one of the phases of processing in SQL-NS applications, described in the section "Component Configuration and the Phases of Processing" (p. 61) in Chapter 3, "The Simplest Notification Application: Stock Quotes."

Figure 5.1. Parts of a SQL-NS application.

Think of the matching component as the core of the SQL-NS application, to which all the other components connect as inputs or outputs. The matching component is where the logic of the application is implemented. The input and output components communicate with the matching component through an exchange of data.